Light modulation schemes used for optical recording or optical communication include a direct modulation scheme for modulating a driving current to directly modulate a light source and an indirect modulation scheme for modulating light from a light source that emits a constant amount of light, using a separately provided light modulator.
The direct modulation scheme is limited in its ability to increase modulation speed due to the presence of a threshold current and a capacitance in the light source. Thus, with an increased transfer rate for an optical pickup or optical communication, an indirect modulation scheme has been demanded which enables an increase in modulation speed.
Light modulators based on the indirect modulation scheme expected to achieve high-speed modulation include a phase modulation type that carries out intensity modulation by interference of light with a phase thereof modulated utilizing electro-optic crystals such as LiNbO3 or KTP and a plasmon coupling type that modulates the amount of transmitted light utilizing coupling between surface plasmon polariton (hereinafter simply referred to SPP) and guided light.
The phase modulation type has widely prevailed as a light modulator for optical communication. However, the electro-optic effect changes refractive index only by a small amount, and therefore an electric field needs to be applied over an optical path of several mm in order to obtain sufficient phase modulation. Thus, miniaturizing modulators of the phase modulation type is difficult. Furthermore, disadvantageously, an electrode for applying an electric field is large in size and thus involves a large parasitic capacitance, hindering high-speed modulation.
The plasmon coupling type includes a light modulator that modulates transmitted light utilizing coupling between SPP localized at an interface between metal and an electro-optic polymer and guided light propagating through a waveguide (see, for example, Patent Literature 1). This light modulator applies an electric field to the electro-optic polymer to manipulate SPP excitation conditions and modulates the transmitted light based on the intensity of the coupling between the guided light and the SPP.
FIG. 30 is a cross-sectional view of a conventional plasmon modulator described in Patent Literature 1.
A plasmon modulator 801 includes a waveguide section and a plasmon excitation section disposed adjacent to the waveguide section. The waveguide section includes a waveguide 802 sandwiched between two coating materials 803. Furthermore, the plasmon excitation section includes a photoelectric material 806 sandwiched between two metal electrodes 805a and 805b. Patent Literature 1 introduces an electro-optic polymer as the photoelectric material 806. The waveguide section and the plasmon excitation section are disposed adjacent to each other via a buffer layer 804.
Guided light propagating through the waveguide 802 and SPP localized at an interface between the metal material 805a and the photoelectric material 806 are present in the plasmon modulator 801. The energy of the guided light couples to and is absorbed by the SPP when a phase matching condition between the guided light and the SPP is met. The wavenumber of the SPP depends on the refractive index of an area around the interface. Thus, by applying an electric field to the photoelectric material 806 to change the refractive index of the photoelectric material 806 based on the electro-optic effect, the plasmon modulator 801 can control the wavenumber of the SPP and thus the degree of coupling between the SPP and the guided light. By controlling the amount of attenuation of the guided light caused by the coupling to the SPP, the plasmon modulator 801 can modulate the intensity of output light transmitted through the plasmon modulator 801.
Furthermore, Patent Literature 2 proposes that a two-dimensional periodic structure be formed in the metal electrode. This allows the use of an electro-optic crystal such as LiNbO3 or KTP which exerts a significant electro-optic effect and which has been unable to be utilized for the structure in Patent Literature 1 due to the high refractive index of the electro-optic crystal.
However, the amount of change in refractive index achieved by the electro-optic effect of the electro-optic polymer is very small, about 0.001, when an electric field of 30 V/um is applied. Thus, a change in the phase matching condition between the guided light and the SPP depending on the presence or absence of an applied electric field is small, and a difference in the amount of attenuation of the guided light resulting from the coupling to the SPP is small. In the conventional art, due to the small difference in the amount of attenuation of the guided light depending on whether or not an applied electric field is present, the degree of modulation of modulated light is disadvantageously low. Furthermore, the degree of modulation can be improved by increasing the length of the modulator, but in this case, the modulator disadvantageously has an increased insertion loss.
Furthermore, even a structure using LiNbO3 or KTP as proposed in Patent Literature 2 involves a small change in refractive index. For example, when LiNbO3 is used, a change in refractive index caused by the electro-optic effect is about 0.0016 in amount when an electric field of 10 V/um, which is a dielectric breakdown field for the LiNbO3 crystal, is applied. Thus, the modulator in Patent Literature 2 involves only a small change in the phase matching condition between the guided light and the SPP, failing to change the degree of coupling between the guided light and the SPP. The modulator in Patent Literature 2 disadvantageously has difficulty in achieving a high degree of modulation.